How to get only the construct signature of a class?

Daniel_X_Moore_68210 0
Viewed 95

I'd like to get the type of a TypeScript class constructor function. In my simple attempts it is always generic Function and not the actual constructor with the parameters as I'd expect.

class BufferedController {
  constructor(id: number) {
    this.id = id
  }

  static defaultBufferSize = 100 as const

  id: number
}

// ?? Just generic Function not the actual constructor
type instanceConstructor = BufferedController["constructor"]

// Class type
type C = typeof BufferedController

// This works fine to get other static properties from the class
type n = C["defaultBufferSize"]
// ?? Also generic Function not the actual constructor
type c = C["constructor"]

What is the correct way to get the type of the constructor function from the class? I'd like to end up with a type like constructor(id: number) => BufferedController

typescript constructor
1 Answers

I think I've got what you want. We'll write a type that gets ONLY the construct signature like this:

type CtorType<T> = T extends { new (...args: infer Args): infer Ret } ? { new (...args: Args): Ret } : never;

A class constructor certainly extends just a construct signature, so we take the arguments and inferred return type and create a new construct signature.

It's the same construct signature in the class constructor except it doesn't have anything else like static members.

Then we can use it like this:

declare const ctor: CtorType<typeof BufferedController>;

ctor.defaultBufferSize // error - doesnt exist

new ctor(0);

Playground

user_182449210
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